1. Field of the Invention
The present invention relates to an AD converting device having a double integral AD converter, a dial-type input device, and a resistance-voltage conversion circuit.
2. Description of the Related Art
FIG. 3 shows a configuration of a commonly-used double integral AD converter 10. The double integral AD converter 10 has a switch 11, an integrator 12, a comparator 13 and a logic block 14. The switch 11 selectively outputs one of an input first integrated voltage Vin and a second integrated voltage −Vref in accordance with a command from the logic block 14. The integrator 12 has an operational amplifier 12a, a resistance 12b, and a capacitor 12c. The operational amplifier 12a has a − terminal to which an output from the switch 11 via the resistance 12b is input and a + terminal which is grounded. The operational amplifier 12a integrates differences between a voltage input to the − terminal and a voltage input to the + terminal and outputs a voltage depending on a result of the integration according to a time constant determined by a resistance value R of the resistance 12b and a capacity C of the capacitor 12c. The comparator 13 has a − terminal to which the output voltage of the integrator 12 is input and a + terminal which is grounded. When the output voltage of the integrator 12 is equal to the ground potential, the comparator 13 outputs a signal. The logic block 14 receives the output of the comparator 13 and a clock. The logic block 14 counts the input clocks and controls the switch 11 based on the counted value (hereinafter, referred to as integral time. The logic block 14 outputs the counted value (integral time) as a digital value when receiving the signal from the comparator 13.
Referring to FIG. 4, an integrating operation of the double integral AD converter 10 will be explained. FIG. 4 shows outputs of the integrator 12 of the double integral AD converter 10 over time. In the double integral AD converter 10, when an AD converting operation is started, the logic block 14 switches the switch 11 such that the first integrated voltage Vin is input to the switch 11 and at the same time, starts to count the integral time. When the switch 11 is switched to a side of the first integral voltage Vin, the integrator 12 performs a first integration until the counted value (integral time) becomes a first counted value T0 (hereinafter, referred to as first integral time T0). During the first integration, the output of the integrator 12 constantly decreases with a slope of −Vin/RC as shown in FIG. 4. When the counted value (integral time) becomes the first counted value T0 (first integral time T0), the logic block 14 switches the switch 11 so as to receive the second integrated voltage −Vref, and, at the same time, starts to newly count an integral time. At the same time with the switching operation, the integrator 12 also starts to perform a second integration. The second integrated voltage −Vref is inversely polarized to the first integrated voltage Vin. Therefore, during the second integration, the output of the integrator 12 constantly increases with a slope of Vref/RC. When the output of the integrator 12 returns to 0, the comparator 13 outputs to the logic block 14 a signal indicating that the output of the integrator 12 returns to 0. When receiving the signal, the logic block 14 outputs the counted value T obtained when the logic block 14 receives the signal (hereinafter, referred to as second integral time T) as an output of the AD converter 10 as a digital value. Since the second integrated voltage −Vref is constant, the second integral time T is a value proportional to the first integrated voltage Vin.
The above double integral operation is shown by the following formula (1). The following formula (2) is obtained by solving the formula (1) and arranging the obtained formula as a formula about T. Since the second integrated voltage −Vref and the counted value T0 are constant values, the second integral time T is proportional to the first integrated voltage Vin. The second integral time T does not depend on the resistance R and the capacity C as shown in the formula (2).
                                                        -                              1                RC                                      ⁢                                          ∫                0                                  T                  0                                            ⁢                                                V                  in                                ⁢                                                                  ⁢                                  ⅆ                  t                                                              +                                    1              RC                        ⁢                                          ∫                0                T                            ⁢                                                V                  ref                                ⁢                                                                  ⁢                                  ⅆ                  t                                                                    =        0                            (        1        )                                T        =                                            V              in                                      V              ref                                ⁢                      T            0                                              (        2        )            
In general, the double integral AD converter performs two integral operations, that is, the first integration in which the integration of the first integrated voltage Vin is performed and the second integration in which the integration of the second integrated voltage −Vref is performed. Therefore, a conversion rate of the double integral AD converter is lower than the other type AD converter but high conversion accuracy can be achieved. Accordingly, the double integral AD converter is used in a digital multi-meter, a digital temperature sensor, or the like.
Japanese Patent Application Publication No. H05-083135 discloses that a second integrated voltage is divided to form a reference voltage and the reference voltage is input to an integrator of a double integral AD converter and a comparator and the reference voltage.
In a dial-type tuning radio, and the like, a dial is operated to switch a received frequency. The dial is, for example, formed by a potentiometer. The potentiometer is, for example, formed by a variable resistance in which a resistance value varies according to a rotational angle. In order to convert a rotational amount of the dial of the potentiometer into a digital output by using the double integral AD converter, it is necessary to convert a resistance value obtained after being varied by the variable resistance of the potentiometer into a voltage and input the voltage to the double integral AD converter.
As shown in FIG. 5, a method for obtaining a first integrated voltage Vin by simply applying an electrical current from a current source 22 to the variable resistance 23 of the potentiometer 21 was considered. However, in this method, if the resistance value Rv of the variable resistance 23 or the current value I of the electrical current applied to the variable resistance varies according to a manufacturing variation of the potentiometer 21 and the current source 22 or a usage environment such as a temperature, the first integrated voltage Vin varies. Therefore, as shown by a broken line in FIG. 4, a slope of the first integration −Vin/RC changes and the output of the integrator 12 in the first integration after the first integral time T0 changes. On the other hand, the second integrated voltage −Vref is substantially constant and a value of a slope of the second integration Vref/RC does not change. Therefore, time until the output from the integrator 12 in the second integration becomes zero, that is, the second integral time changes, for example, from T to T′ as shown in FIG. 4. That is, there is a problem in that, even when the rotational amount of the dial of the potentiometer 21 is the same, the second integral time varies according to the manufacturing variation or the usage environment.
This is shown by the following formula (3). That is, when Rv indicates the resistance value of the variable resistance 23 of the potentiometer 21, x indicates the rotational amount (rate of the rotation) of the potentiometer 21, and I indicates the electrical current applied from the current source 22, the first integrated voltage Vin is shown by the formula (3). The formula (4) is obtained by substituting the formula (3) into the formula (1). That is, as shown in the formula (3), even if the value of x is constant, the value of the second integral time T (digital output) varies when the electrical current I applied from the current source 22 and/or the resistance value Rv of the variable resistance 23 of the potentiometer 21 varies.
                              V          in                =                  x          ⁢                                          ⁢                      R                          v              ⁢                                                                            ⁢          I          ⁢                                          ⁢                      (                          0              ≤              x              ≤              1                        )                                              (        3        )                                T        =                                            x              ⁢                                                          ⁢                              R                v                            ⁢              I                                      V              ref                                ⁢                      T            0                                              (        4        )            